1. Field of the Invention
The invention relates to a vehicle differential gear that distributes a driving force of a drive source of a vehicle to a pair of output shafts while allowing a differential operation.
2. Description of Related Art
Conventionally, a vehicle differential gear that distributes a driving force of a drive source of a vehicle to a pair of output shafts while allowing a differential operation is, for example, arranged between right and left wheels of the vehicle. As such a vehicle differential gear, a vehicle differential gear is known, in which the differential between the output shafts is limited so that even if slip occurs at a wheel to which the driving force is transmitted by one output shaft, e.g., a left wheel, the driving force can be transmitted to a wheel to which the driving force is transmitted by the other output shaft, e.g., a right wheel, so that a travelling stability during travelling on a low μ road and the like is enhanced. See for example, Japanese Patent Application Publication No. 2009-174577 (JP 2009-174577 A).
The vehicle differential gear (differential gear mechanism) described in JP 2009-174577 A includes a differential case (housing), to which a driving force of an engine is input, a plurality of pairs of pinion gears (planet gears) rotatably supported by the differential case, a pair of side gears (sun gears) that mesh with the plurality of pairs of pinion gears, a center washer arranged between opposing surfaces of the pair of side gears, and an end washer arranged between each of the pair of side gears and the differential case. A lubricating oil is introduced into the housing, so that the sliding of each unit is lubricated by the lubricating oil.
The pinion gear and the side gear are helical gears having helical teeth, where a thrust force in an axial direction generates between the respective gears due to meshing of the pinion gear and the side gear. This thrust force causes the pair of side gears to be pushed against the center washer or the end washer thus generating a frictional force, and such frictional force limits the differential between the pair of side gears.
With an increase in environmental awareness in recent years, further enhancement in fuel efficiency performance of the vehicle is desired, and downsizing and weight reduction are also demanded on the vehicle differential gear. However, if the pinion gear, the side gear, and the washers (center washer and end washer) forming the vehicle differential gear are simply downsized, an area of the sliding surfaces of the side gear and the washer decreases, thus increasing a surface pressure due to the thrust force. When the side gear is rotated at a high speed with respect to the washer in a high surface pressure state, the possibility that an oil film shortage locally occurs becomes higher. If the oil film shortage occurs in such manner, a friction coefficient at the sliding surface increases and the frictional force becomes unstable, whereby vibration may occur during the relative rotation of the pair of side gears.